Wheelock shows the future active participle because the normal 4th principal part, the perfect passive participle, is not in use (due to the intransitive nature of the verb). The future active participle, however, is in use, so it is more useful to the reader to know the supine stem after all, rather than to just leave it without a 4th principal part. F&W and many other references are a little bit misleading by showing you the unused perfect passive participle for certain verbs like this, but their intention is just to give you the supine stem for the construction of future active participles and impersonal passives.